Prior art photodiodes are operative to convert optical signals into electrical signals. For example, prior art photodiodes are commonly used in the fiber-optic industry where they are placed at the end of an optical fiber communications network and serve as the interface between the electrical and optical circuits. An avalanche photodiode is a type of prior art photodiode that has a very high light sensitivity, and that can provide extremely accurate electrical signal representations from very small inputted light signals. The prior art avalanche photodiode may produce an increase of ten or more decibels of sensitivity over other prior art means for converting optical signals to electrical signals. The added sensitivity provided by the prior art avalanche photodiode enables increases of optical length and/or bandwidth.
Prior art avalanche photodiodes typically rely on the effect of a voltage breakdown across a reverse-biased p-n biased end junction. This voltage breakdown produces holes and electrons within the substrate of the prior art avalanche photodiodes.
The prior art avalanche photodiode uses both a special doping profile and a high voltage bias to achieve electrical signal amplification within a light sensitive material. The incoming photons of light operate to free electrons within the semi-conducting substrate of the prior art avalanche photodiode. The high voltage bias of the prior art avalanche photodiode then sweeps these electrons so rapidly through the special doping profile of its substrate that collisions between electrons and the semi-conductive substrate in the avalanche photodiode serve to liberate additional electrons. This process is self-multiplying, such that a few input photons of light cause many electrons to flow as the output of the prior art avalanche photodiode.
The avalanche process within a prior art avalanche photodiode is temperature dependent, and hence temperature stabilization methods often are required in prior art avalanche photodiode applications. Prior art avalanche photodiodes have not been extensively used in military or aerospace applications, because these environments tend to have extreme temperatures which may adversely affect the avalanche photodiode biasing point.
Prior art stabilization circuits for avalanche photodiodes typically include a controllable constant voltage source with current limiting and a matched pair of avalanche photodiodes. The voltage drop across one avalanche photodiode acts as a bias reference. This voltage source then is used to bias the other photodiode in the prior stabilization circuit. However, tracking between the two diodes is imperfect and is unable to optimize optical sensitivity in extreme temperature conditions, such as the temperature conditions encountered in military and aerospace environments. Thus, avalanche photodiodes which rely on the prior art stabilization circuit do not achieve optimum performance at temperature extremes. Moreover, matching a pair of photodiodes for bias purposes, as in the prior art stabilization circuit, is both difficult and expensive. Hence, the prior art stabilization circuit for the avalanche photodiode was prone to numerous deficiencies.
Accordingly, an object of the subject invention is to avoid self-destruction of the prior art avalanche photodiode in environments subject to a wide temperature range by limiting diode current.
It is a further object of the subject invention to modulate the operating point of the avalanche photodiode and thereby making the high sensitivity of avalanche photodiodes usable over a wide range of electrical and environmental conditions.
A further object of the present invention is to provide a means in which the photodiode is to be utilized in extreme temperature environments.